Evidence from depressed patients indicates the neurotransmission in the central nervous system (CNS) may be disturbed. These disturbances involve the neurotransmitters noradrenaline (NA) and 5-hydroxytryptamine (5-HT). The drugs most frequently used in the treatment of depression are considered to act by improving the neurotransmission of either or both of these physiological agents. The mechanism of action for conventional drugs used to treat mental depression is generally believed to be indirect. It is thought the drugs block the reuptake of the neurotransmitters released from nerve terminals in the CNS, NA and/or 5-HT, which increases the concentration of these transmitters in the synaptic cleft and restores an adequate neurotransmission. For example, the clinically documented antidepression drug, zimelidine (dimethyl-amino-1-(4-bromophenyl)-1-(3-pyridyl)propene) acts as such a reuptake inhibitor with high selectivity for 5-HT neurons.
Available data suggests enhancement of 5-HT neurotransmission will primarily improve depressed mood and anxiety, whereas enhancement of noradrenaline neurotransmission will improve the retardation symptoms occurring in depressed patients. In recent years many efforts have been made to develop new drugs with high selectivity for the improvement of the 5-HT neurotransmission in the CNS.
A fundamentally different way to improve the neurotransmission in the central 5-HT neurons would be to use a 5-HT receptor agonist acting directly upon the 5-HT receptors, and particularly the 5-HT.sub.1A receptor. In order to minimize undesired side effects, a high selectivity for this kind of receptor would be necessary.
Clinically, 5-HT.sub.1A agonists have also demonstrated anxiolytic properties. The drug, Buspirone, is the only currently available marketed 5-HT.sub.1A agonist having anxiolytic activity. This compound antagonizes dopamine receptors at the same dose it stimulates 5-HT.sub.1A receptors. These dopamine antagonist properties reduce the clinical utility of these compounds however because long term treatment with dopamine antagonists can produce tardive dyskinesias.
The search for new CNS active compounds is focused on finding compounds with selective 5-HT.sub.1A receptor agonist effects without detrimentally influencing central dopamine receptors.
In recent years a large body of pharmacological, biochemical and electrophysical evidence has provided considerable support in favor of the existence of a specific population of central autoregulatory dopamine receptors located in the dopaminergic neuron itself and belonging to the D.sub.2 receptor subclass of dopamine receptors. These receptors are part of a homeostatic mechanism that modulates nerve impulse flow and transmitter synthesis and regulates the amount of dopamine released from the nerve endings.
Drugs acting on central dopamine transmission are clinically effective in treating a variety of central nervous system disorders such as parkinsonism and schizophrenia. In parkinsonism, for example, the nigro-neostriatal hypofunction can be restored by an increase in postsynaptic dopamine receptor stimulation. In schizophrenia, the condition can be normalized by achieving a decrease in postsynaptic dopamine receptor stimulation. Classical antipsychotic agents directly block the postsynaptic dopamine receptor. The same effect can be achieved by inhibition of intraneuronal presynaptic events essential for the maintenance of adequate neurotransmission, transport mechanism and transmitter synthesis.
Direct dopamine receptor agonists, like apomorphine, are able to activate the dopamine autoreceptors as well as the postsynaptic dopamine receptors. The effects of autoreceptor stimulation appear to predominate when apomorphine is administered at low doses, whereas at higher doses the attenuation of dopamine transmission is outweighed by the enhancement of postsynaptic receptor stimulation. The antipsychotic and antidyskinetic effects in man of low doses of apomorphine are likely due to the autoreceptor-stimulator properties of this dopamine receptor agonist. This body of knowledge indicates dopamine autoreceptors would be valuable in treating psychiatric disorders.